Fuel cell system with humidity determination

ABSTRACT

In a polymer electrolyte fuel cell system, a condition of humidification of electrolyte membranes is precisely determined, and adjustment is performed so that the condition of humidification of the electrolyte membranes substantially remains within a proper range. The system determines a condition of humidification of the electrolyte membranes of the fuel cell based on a differential value of the resistance of the fuel cell detected by a resistance detector when the electric current outputted by the fuel cell equals a predetermined value. The current from the cell is detected by an ammeter. The fuel cell is formed by stacking electrically conductive electrodes and separators, and proton-conductive electrolyte membranes. The electric conductivity of the electrodes and the separators is not affected by the humidification condition thereof, whereas the proton-conductivity of the electrolyte membranes greatly changes depending on the condition of humidification thereof. Therefore, the changing rate of the resistance of the fuel cell directly represents the changing rate of the proton-conductivity of the electrolyte membranes, so that the humidification condition of the electrolyte membranes can be precisely determined.

The disclosure of Japanese Patent Application No. HEI 11-44200 filed onFeb. 23, 1999 including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fuel cell system and, moreparticularly, to a fuel cell system having a polymer electrolyte fuelcell formed by stacking unit cells, each of which has an electrolytemembrane and two electrodes sandwiching the electrolyte membrane.

2. Description of the Related Art

A fuel cell system that determines, based on the humidity in a polymerelectrolyte fuel cell and the voltage outputted by the fuel cell,whether the amount of moisture or water contained in electrolytemembranes is insufficient, and a fuel cell system that determineswhether the amount of water contained in electrolyte membranes of a fuelcell is sufficient based on an electric current threshold derived byusing the amount of change in voltage and temperature as parameters, andon the current outputted by the fuel cell (described in, for example,Japanese Patent Application Laid-Open No. HEI 7-272736) have beenproposed. These systems derive an allowable minimum voltage, as athreshold, corresponding to a detected reaction temperature of the fuelcell by using a table that indicates a relationship between theallowable minimum voltage outputted from the fuel cell and the reactiontemperature of the fuel cells. If the voltage outputted from the fuelcell is less than the threshold, it is determined that the amount ofelectrolyte membrane-contained water is insufficient. The systems alsoderive a maximum electric current value as a threshold by using theamount of change in the voltage outputted from the fuel cell and thetemperature of the fuel cells as parameters. If the current outputtedfrom the fuel cell is greater than the threshold, it is determined thatthe amount of electrolyte membrane-contained water is insufficient. Whenit is determined that the amount of electrolyte membrane-contained wateris insufficient, the systems limit the current supplied to loads.

However, the above-described fuel cell systems have a drawback ofreduced precisions in determining the amount of electrolytemembrane-contained water because the amount of electrolytemembrane-contained water considerably changes depending on the fuel celltemperature, the fuel gas pressure, the amount of fuel gas supplied, andthe like. Furthermore, when it is determined that the amount ofelectrolyte membrane-contained water is insufficient, the systems limitthe current supplied to loads, so as to protect the electrolytemembranes from damage. However, it is difficult to achieve anappropriate range of the amount of electrolyte membrane-contained waterin the systems.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to precisely determine acondition of humidification of electrolyte membranes in a fuel cellsystem. It is another object to perform adjustment such that thecondition of humidification remains within a proper range. It is stillanother object to determine that there is an abnormality in a fuel cellsystem if the condition of humidification of electrolyte membranescannot be adjusted to a proper range and to protect the electrolytemembranes from damage when it is determined that there is anabnormality.

To achieve at least one of the aforementioned and other objects of theinvention, one aspect of the invention provides a fuel cell systemhaving a polymer electrolyte type fuel cell formed by stacking unitcells, each of which has an electrolyte membrane sandwiched by twoelectrodes, the system including a fuel gas supplier that supplies afuel gas to the fuel cell, a fuel gas humidifier that humidifies thefuel gas, a current detector that detects an electric current outputtedfrom the fuel cell, a resistance detector that detects a resistance ofthe fuel cell, and a humidification condition determiner that determinesa condition of humidification of the electrolyte membranes based on thecurrent detected by the current detector and the resistance detected bythe resistance detector.

This fuel cell system determines a condition of humidification of theelectrolyte membranes based on the current outputted by the fuel celland the electrical resistance of the fuel cell, which is directlydetermined by the condition of humidification of the electrolytemembranes. Therefore, the system provides precise determinationregarding the condition of humidification of the electrolyte membranes.

Another aspect of the invention provides a fuel cell system having apolymer electrolyte type fuel cell formed by stacking unit cells, eachof which has an electrolyte membrane sandwiched by two electrodes, thesystem including a fuel gas supplier that supplies a fuel gas to thefuel cell, a fuel gas humidifier that humidifies the fuel gas, a currentdetector that detects an electric current outputted from the fuel cell,a voltage detector that detects a voltage outputted from the fuel cell,a fuel gas supply changer that changes an amount of the fuel gassupplied to the fuel cell, a humidification condition determiner thatdetermines a condition of humidification of the electrolyte membranesbased on the current and the voltage detected by the current detectorand the voltage detector, respectively, when the amount of the fuel gassupplied is changed by the fuel gas supply changer.

This fuel cell system determines a condition of humidification of theelectrolyte membranes based on the current, the voltage, and the amountof the fuel gas supplied to the fuel cell, which is a factor thataffects the condition of humidification of the electrolyte membranes.Therefore, the system provides precise determination regarding thecondition of humidification of the electrolyte membranes.

Still another aspect of the invention provides a fuel cell system havinga polymer electrolyte type fuel cell formed by stacking unit cells, eachof which has an electrolyte membrane sandwiched by two electrodes, thesystem including a fuel gas supplier that supplies a fuel gas to thefuel cell, a fuel gas humidifier that humidifies the fuel gas, a voltagedetector that detects a voltage outputted from the fuel cell, a fuel gassupply changer that changes an amount of the fuel gas supplied to thefuel cell, and a humidification condition determiner that determines acondition of humidification of the electrolyte membranes based on afirst voltage detected by the voltage detector before the amount of thefuel gas supplied is changed by the fuel gas supply changer and a secondvoltage detected by the voltage detector after the amount of the fuelgas supplied is changed by the fuel gas supply changer.

This fuel cell system determines a condition of humidification of theelectrolyte membranes based on the voltage and the amount of the fuelgas supplied to the fuel cell, which is a factor that affects thecondition of humidification of the electrolyte membranes. Therefore, thesystem provides precise determination regarding the condition ofhumidification of the electrolyte membranes.

A further aspect of the invention provides a fuel cell system having apolymer electrolyte type fuel cell formed by stacking unit cells, eachof which has an electrolyte membrane sandwiched by two electrodes, thesystem including a fuel gas supplier that supplies a fuel gas to thefuel cell, a fuel gas humidifier that humidifies the fuel gas, a voltagedetector that detects a voltage outputted from the fuel cell, a fuel gassupply changer that changes an amount of the fuel gas supplied to thefuel cell, a humidification condition determiner that determines acondition of humidification of the electrolyte membranes based on afirst variation of voltages detected by the voltage detector before theamount of the fuel gas supplied is changed by the fuel gas supplychanger and a second variation of voltages detected by the voltagedetector after the amount of the fuel gas supplied is changed by thefuel gas supply changer.

In this fuel cell system, the voltage detector may detect a voltage ofeach unit cell of the fuel cell, or may detect a voltage of each modulethat has at least two of the unit cells. A change in the amount of thefuel gas supplied to the fuel cell affects the condition ofhumidification of the electrolyte membranes. When the condition ofhumidification of the electrolyte membranes changes, the change appearsin the variation (e.g., variance) of the voltages of the individual unitcells or the individual fuel cell modules.

Therefore, by determining a condition of humidification of theelectrolyte membranes based on the voltages of the unit cells or thefuel cell modules, which reflects the condition of humidification of theelectrolyte membranes, the system is able to provide precisedetermination regarding the condition of humidification of theelectrolyte membranes.

In the above-described aspects of the invention, a degree ofhumidification in the fuel gas may be controlled based on thedetermination made by the humidification condition determiner. In thismanner, the condition of humidification of the electrolyte membranes canbe adjusted.

Furthermore, an abnormality of the fuel cell system may be detected ifthe condition of humidification of the electrolyte membranes does notchange despite the control of the condition of humidification performedfor a predetermined amount of time. In order to provide notification ofthe occurrence of an abnormality, information regarding the abnormalitymay be outputted. The operation of the fuel cell system may also bestopped upon detection of an abnormality. Therefore, it becomes possibleto protect the fuel cell from damage or the like that can possibly occurduring operation in the presence of an abnormality.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further objects, features and advantages of thepresent invention will become apparent from the following description ofa preferred embodiment with reference to the accompanying drawings,wherein like numerals are used to represent like elements and wherein:

FIG. 1 is a schematic illustration of a construction of fuel cell systemaccording to the invention;

FIG. 2 is a schematic illustration of a construction of a unit cell of afuel cell;

FIG. 3 is a flowchart illustrating a humidification control routine;

FIG. 4 is a flowchart illustrating a humidification conditiondetermining routine;

FIG. 5 is a flowchart illustrating another humidification conditiondetermining routine;

FIG. 6 is a flowchart illustrating still another humidificationcondition determining routine; and

FIG. 7 is a flowchart illustrating a further humidification conditiondetermining routine.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A preferred embodiment of the invention will be described hereinafterwith reference to the accompanying drawings.

FIG. 1 is a schematic illustration of a construction of a fuel cellsystem according to the invention. A fuel cell system 20 has a polymerelectrolyte fuel cell 30 that generates electric power when suppliedwith a fuel gas and an oxidative gas, a fuel gas supplier device 22 forsupplying the fuel gas containing hydrogen to the fuel cell 30, a fuelgas humidifier 23 for humidifying the fuel gas, an oxidative gassupplier device 24 for supplying the oxidative gas containing oxygen(e.g., air) to the fuel cell 30, an oxidative gas humidifier 25 forhumidifying the oxidative gas, a cooling device 50 for cooling the fuelcell 30, and an electronic control unit 60 for controlling the operationof the fuel cell system 20.

The fuel gas supplier device 22 may be, for example, a reformer thatreforms a hydrocarbon fuel, such as methanol, methane or the like, intoa hydrogen-rich fuel gas and supplies the hydrogen-rich fuel gas to thefuel cell 30, or a fuel gas storage tank that stores ahydrogen-containing fuel gas. The oxidative gas supplier device 24 maybe an air pump that simply supplies air, or an oxidative gas storagetank that stores an oxidative gas other than air. The fuel gas supplierdevice 22 and the oxidative gas supplier device 24 are connected to theelectronic control unit 60 by signal lines, so that the electroniccontrol unit 60 controls the amount of fuel gas supplied and the amountof oxidative gas supplied.

The fuel gas humidifier 23 and the oxidative gas humidifier 25 vaporizewater pumped from a water tank 26 and supply water vapor to the fuel gasand the oxidative gas. The fuel gas humidifier 23 and the oxidative gashumidifier 25 are connected to the electronic control unit 60 by signallines, so that the electronic control unit 60 controls the amount ofmoisture or the degree of humidification in the fuel gas and the amountof moisture or the degree of humidification in the oxidative gas.

The fuel cell 30 is a polymer electrolyte fuel cell formed by stacking aplurality of unit cells 31. FIG. 2 is a schematic illustration of a unitcell 31 of the fuel cell 30. As shown in FIG. 2, a unit cell 31 has anelectrolyte membrane 32 that is a proton-conductive membrane formed froma polymer material such as fluorocarbon resin or the like, and an anode33 and a cathode 34 which are gas diffusion electrodes formed by acarbon cloth carrying a platinum catalyst or an alloy catalyst formed byplatinum and one or more other metals. The anode 33 and the cathode 34sandwich the electrolyte membrane 32 so that the electrolyte membrane 32is held by catalyst-carrying side faces of the anode 33 and the cathode34. The unit cell 31 further includes two separators 35 disposed onopposite sides of the above-described sandwich structure. Together withanode 33 and the cathode 34, the separators 35 define channels 36, 37for the fuel gas and the oxidative gas. The separators 35 also serve aspartitions between adjacent unit cells 31.

The fuel cell 30 is equipped with a voltmeter 40 and an ammeter 42 fordetecting the voltage V and the current I outputted from the fuel cell30, a fuel cell temperature sensor 44 for detecting the temperature ofthe fuel cell 30, a pressure sensor 46 for detecting the gas pressure Pof the fuel gas and the oxidative gas, a resistance detector 48 fordetecting the electrical resistance of the fuel cell 30, and the like.These sensors and the like are connected to the electronic control unit60 by signal lines. An example of the method for detecting theresistance of the fuel cell 30 is one in which the resistance of thefuel cell 30 is determined from a value of current from the fuel cell 30occurring when an AC voltage is applied to an output terminal of thefuel cell 30. The resistance of the fuel cell 30 can be roughly dividedinto the resistances of the anodes 33, the cathodes 34 and theseparators 35, and the resistance based on the proton conductivity ofthe electrolyte membranes 32. The resistances of the anodes 33, thecathodes 34 and the separators 35 remain unchanged regardless of whetherthe components are humidified, because the anodes 33, the cathodes 34and the separators 35 are formed from electrically conductive materials.In contrast, the proton conductivity of the electrolyte membranes 32remarkably changes depending on whether the electrolyte membranes 32 arewet. Therefore, the value of resistance of the fuel cell 30 reflects thedegree of wetness of the electrolyte membranes 32.

Fuel gas and oxidative gas discharge pipes of the fuel cell 30 areprovided with pressure-regulating valves 27, 28, so that the fuel gaspressure and the oxidative gas pressure in the fuel cell 30 can beadjusted. Actuators 27 a, 28 a of the pressure-regulating valves 27, 28are connected to the electronic control unit 60 by signal lines, so thatthe actuators 27 a, 28 a are driven and controlled by the electroniccontrol unit 60.

The cooling device 50 has a cooling plate disposed inside the fuel cell30, a cooling water channel 52 that forms a cooling water passage formedin the cooling plate and a circulation channel, a heat exchanger 56disposed in the cooling water channel 52 for cooling the cooling waterthrough heat exchange with the external air, a cooling water pump 54 forcirculating the cooling water through the circulation channel, and acooling water temperature sensor 58 for detecting the temperature of thecooling water near an outlet opening of the fuel cell 30. The coolingwater pump 54 and the cooling water temperature sensor 58 are connectedto the electronic control unit 60 by signal lines, so that theelectronic control unit 60 controls the cooling of the fuel cell 30.That is, based on the cooling water temperature detected by the coolingwater temperature sensor 58, the electronic control unit 60 drives thecooling water pump 54 so as to control the flow of circulating coolingwater.

The electronic control unit 60 is formed as a one-chip microprocessorhaving a CPU 62 as a main component. The electronic control unit 60further has a ROM 64 storing processing programs, a RAM 66 fortemporarily storing data, and input and output ports (not shown). Thetemperatures and flows of fuel gas and oxidative gas supplied from thefuel gas supplier device 22 and the oxidative gas supplier device 24 areinputted to the electronic control unit 60 from thermometers (not shown)and flowmeters (not shown ) via input ports. The electronic control unit60 further receives information regarding the operational conditions ofthe fuel gas humidifier 23 and the oxidative gas supplier device 24, thevoltage V from the voltmeter 40, the current I from the ammeter 42, thetemperature from the fuel cell temperature sensor 44, the gas pressure Pfrom the pressure sensor 46, the value of resistance from the resistancedetector 48, the temperature from the cooling water temperature sensor58, and the like via input ports. The electronic control unit 60 outputsdrive signals to the fuel gas supplier device 22, the oxidative gassupplier device 24, the fuel gas humidifier 23, the oxidative gashumidifier 25 and the cooling water pump 54, and outputs a lightingsignal to an indicator lamp 68, and the like, via output ports.

The operation of the fuel cell system 20 constructed as described aboveand, in particular, the control of humidification of the fuel cell 30,will be described. FIG. 3 is a flowchart illustrating a humidificationcontrol routine executed by the electronic control unit 60 of the fuelcell system 20. This routine is executed at every predetermined timeduring a period that starts immediately after the operation of the fuelcell system 20 is started and that ends when the operation of the fuelcell system 20 is stopped.

When the humidification control routine is started, the CPU 62 executesa process of determining a condition of humidification of theelectrolyte membranes 32. The determining process of step S100 isperformed by a humidification condition determining routine (describedin detail later) as illustrated in any of FIGS. 4 through 7. Thedetermining routine outputs one of three results, that is, “properhumidification”, “insufficient humidification”, and “excesshumidification”.

If the determination made by the humidification condition determiningprocess of step S100 is “proper humidification”, the CPU 62 resets acounter C1 and a counter C2 in steps S104 and S106. If the determinationis “insufficient humidification”, the CPU 62 executes a process fordiscontinuing the insufficient humidification of the electrolytemembranes 32 in step S108, and increments the counter C1 in step S110.

Examples of the process for discontinuing insufficient humidification ofthe electrolyte membranes 32 include:

(A1) a process of increasing the degrees of humidification in the fuelgas and the oxidative gas by using the fuel gas humidifier 23 and theoxidative gas humidifier 25;

(A2) a process of raising the gas pressures P of the fuel gas and theoxidative gas in the fuel cell 30 by closing the pressure regulatingvalves 27, 28;

(A3) a process of lowering the operating temperature of the fuel cell 30by increasing the flow of cooling water circulated by the cooling waterpump 54 of the cooling device 50; and

(A4) a process of reducing the amounts of the fuel gas and the oxidativegas supplied to the fuel cell 30 from the fuel gas supplier device 22and the oxidative gas supplier device 24.

The process (A1) directly discontinues insufficient humidification ofthe electrolyte membranes 32 by increasing the degrees of humidificationin the fuel gas and the oxidative gas. The process (A2) discontinuesinsufficient humidification of the electrolyte membranes 32 on the basisof a fact that increased gas pressures increase the water vapor pressurein the fuel gas and the oxidative gas. The process (A3) discontinuesinsufficient humidification of the electrolyte membranes 32 on the basisof a fact that a lowered operating temperature of the fuel cell 30lowers the fuel gas temperature and the oxidative gas temperature and,therefore, the water vapor pressures in the fuel gas and the oxidativegas increase. The process (A4) discontinues insufficient humidificationof the electrolyte membranes 32 on the basis of a fact that reducedamounts of the fuel gas and the oxidative gas supplied reduce waterevaporation from the electrolyte membranes 32. From such variousprocesses for discontinuing insufficient humidification of theelectrolyte membranes 32, either a single process or a combination oftwo or more processes may be adopted. It is also possible tosequentially change the process or processes to be performed for everyexecution cycle of the humidification control routine.

If the humidification condition determined in step S102 is “excesshumidification”, the CPU 62 executes a process for discontinuing excesshumidification of the electrolyte membranes 32 in step S112, andincrements the counter C2 in step S114.

Examples of the process for discontinuing excess humidification of theelectrolyte membranes 32 include:

(B1) a process of reducing the degrees of humidification in the fuel gasand the oxidative gas by using the fuel gas humidifier 23 and theoxidative gas humidifier 25;

(B2) a process of lowering the gas pressures P of the fuel gas and theoxidative gas in the fuel cell 30 by opening the pressure regulatingvalves 27, 28;

(B3) a process of raising the operating temperature of the fuel cell 30by decreasing the flow of cooling water circulated by the cooling waterpump 54 of the cooling device 50; and

(B4) a process of increasing the amounts of the fuel gas and theoxidative gas supplied to the fuel cell 30 from the fuel gas supplierdevice 22 and the oxidative gas supplier device 24.

The process (B1) directly discontinues excess humidification of theelectrolyte membranes 32 by decreasing the degrees of humidification inthe fuel gas and the oxidative gas. The process (B2) discontinues excesshumidification of the electrolyte membranes 32 on the basis of a factthat decreased gas pressures decrease the water vapor pressures in thefuel gas and the oxidative gas. The process (B3) discontinues excesshumidification of the electrolyte membranes 32 on the basis of a factthat a raised operating temperature of the fuel cell 30 raises the fuelgas temperature and the oxidative gas temperature and, therefore, thewater vapor pressures in the fuel gas and the oxidative gas decrease.The process (B4) discontinues excess humidification of the electrolytemembranes 32 on the basis of a fact that increased amounts of the fuelgas and the oxidative gas supplied accelerate water evaporation from theelectrolyte membranes 32. From such various processes for discontinuingexcess humidification of the electrolyte membranes 32, either a singleprocess or a combination of two or more processes maybe adopted. It isalso possible to sequentially change the process or processes to beperformed for every execution cycle of the humidification controlroutine.

The count of the counter C1 is incremented when the process fordiscontinuing insufficient humidification of the electrolyte membranes32 has been sequentially performed. Likewise, the counter C2 indicatesthe number of times that the process for discontinuing excesshumidification has been performed sequentially.

After incrementing the count C1 or C2 subsequently to the process fordiscontinuing insufficient humidification or excess humidificationcorresponding to the result of determination regarding thehumidification condition of the electrolyte membranes 32, the CPU 62compares the count C1, C2 with a predetermined time threshold Cref instep S116. The threshold Cref is preset on the basis of the number oftimes of executing the process or the amount of time that is needed inorder to consider that the process for discontinuing insufficient orexcess humidification of the electrolyte membranes 32 has beensufficiently performed. Therefore, if the count C1 or C2 is equal to orgreater than the threshold Cref, the CPU 62 determines that anabnormality has occurred in the fuel cell system 20, and turns on theindicator lamp 68 to inform an operating person of the abnormality instep S118. Subsequently in step S120, the CPU 62 stops the operation ofthe fuel cell system 20 in order to protect the fuel cell 30 and thelike. This routine then ends. Conversely, if both the counts C1, C2 areless than the threshold Cref, the CPU 62 determines that there is noabnormality in the fuel cell system 20, and then ends the routine.

As is apparent from the above description, the fuel cell system 20according to the invention is able to control the condition ofhumidification of the electrolyte membranes 32 on the basis of theresult of determination regarding the condition of humidification of theelectrolyte membranes 32, by executing the humidification controlroutine illustrated in FIG. 3. For example, insufficient humidificationor excess humidification of the electrolyte membranes 32 can bediscontinued by any one or more of the processes (A1)-(A4) or theprocesses (B1)-(B4).

Furthermore, by executing the humidification control routine illustratedin FIG. 3, the fuel cell system 20 is able to determine that anabnormality has occurred in the system if insufficient humidification orexcess humidification is not discontinued despite repeated performanceof the process for discontinuing insufficient or excess humidificationof the electrolyte membranes 32. In such a case, the fuel cell system 20informs an operating person of the occurrence of an abnormality andstops the operation of the fuel cell system 20. Therefore, the fuel cellsystem 20 makes it possible for an operating person to quickly realizeoccurrence of an abnormality and is able to prevent, for example, damageto the fuel cell system 20 that can be caused if the operation of thefuel cell system 20 is continued in the presence of an abnormality

The processing of step S100 in FIG. 3, that is, the process ofdetermining a condition of humidification of the electrolyte membranes32, will now be described. FIGS. 4 to 7 are flowcharts illustratingexamples of a humidification condition determining routine thatdetermines a condition of humidification of the electrolyte membranes32. Any one or more of the four humidification condition determiningroutines may be executed in step S100 in FIG. 3. It is also possible tosequentially change the process or processes to be performed for everyexecution cycle of the humidification control routine illustrated inFIG. 3.

The humidification condition determining routines illustrated in FIGS. 4to 7 will be individually described below.

When the humidification condition determining routine illustrated inFIG. 4 is started, the CPU 62 reads, in step S200, an output current Iof the fuel cell 30 detected by the ammeter 42, and a resistance R ofthe fuel cell 30 detected by the resistance detector 48. Subsequently instep S202, the CPU 62 compares the current I with a predeterminedcurrent Iset. If the current I does not equal the predetermined currentIset, the CPU 62 returns to step S200. The predetermined current Iset ispreset to an ordinary current value that is not particularly great norsmall. Therefore, the current I outputted from the fuel cell 30frequently equals the predetermined current Iset.

When the current I equals the predetermined current Iset, the CPU 62substitutes the resistance R detected in step S200 for the previousresistance R0 in step S204. Subsequently in step S206, the CPU 62 readsa resistance R of the fuel cell 30 detected by the resistance detector48. In step S208, the CPU 62 sets, as a differential value dR/dt, avalue obtained by dividing the difference of the presently readresistance R from the previous resistance R0 by an infinitesimal timeAt. The infinitesimal time Δt is set as a time consumed between thereading of the resistance R performed in step S200 when the current Iequals the predetermined current Iset and the reading of the resistanceR in step S206. The infinitesimal time At is determined in accordancewith the performance of the CPU 62 of the electronic control unit 60,and the contents of the processings that are parallelly executed by theCPU 62, and the like.

After the calculation in step S208, the CPU 62 determines in step S210whether the differential value dR/dt is within a proper humidificationrange set between a predetermined resistance differential threshold aand a predetermined resistance differential threshold β. When thehumidification of the electrolyte membranes 32 is insufficient, theproton conductivity of the electrolyte membranes 32 decreases, so thatthe response to small changes of the humidification condition of theelectrolyte membranes 32 becomes dull and, therefore, the differentialvalue dR/dt becomes small. Conversely, when the humidification of theelectrolyte membranes 32 is excessive, the proton conductivity of theelectrolyte membranes 32 increases, so that the response to changes ofthe humidification condition of the electrolyte membranes 32 is quickeven if such a change is small and, therefore, the differential valuedR/dt becomes great. Therefore, the humidification condition of theelectrolyte membranes 32 can be determined by determining a differentialvalue dR/dt when the condition of humidification of the electrolytemembranes 32 is within the proper range, and setting a properhumidification range through the use of the threshold a and thethreshold β as a lower limit and an upper limit, respectively, andcomparing the calculated differential value dR/dt with the properhumidification range.

When the differential value dR/dt is within the proper humidificationrange, the CPU 62 determines in step S212 that the humidification of theelectrolyte membranes 32 is proper (proper humidification) When thedifferential value dR/dt is equal to or less than the threshold a, theCPU 62 determines in step S214 that the humidification of theelectrolyte membranes 32 is insufficient (insufficient humidification).When the differential value dR/dt is equal to or greater than thethreshold β, the CPU 62 determines in step S216 that the humidificationof the electrolyte membranes 32 is excessive (excess humidification).This routine then ends.

By executing the humidification condition determining routineillustrated in FIG. 4, the fuel cell system 20 is able to determine ahumidification condition of the electrolyte membranes 32 based on thecurrent I outputted from the fuel cell 30 and the resistance R of thefuel cell 30. Furthermore, since the resistance R of the fuel cell 30directly reflects the humidification condition of the electrolytemembranes 32, the humidification condition of the electrolyte membranes32 can be more accurately determined.

Next described will be a humidification condition determining routineillustrated in FIG. 5. When this routine is started, the CPU 62 firstincreases the amount of oxidative gas supplied from the oxidative gassupplier device 24 to the fuel cell 30 in step S300. Subsequently, theCPU 62 repeats the sequence of step S301 of reading the current Idetected by the ammeter 42 and the voltage V detected by the voltmeter40 and step S302 of comparing the read current I with a predeterminedcurrent Iset, until the detected current I equals the predeterminedcurrent Iset. The predetermined current Iset is the same as thepredetermined current Iset used in the humidification conditiondetermining routine illustrated in FIG. 4.

When the detected current I equals the predetermined current Iset, theCPU 62 substitutes the voltage V detected in step S301 for the previousvoltage V0 in step S304. Subsequently in step S306, the CPU 62 reads avoltage V detected by the voltmeter 40. In step S308, the CPU 62 sets,as a differential value dV/dt, a value obtained by dividing thedifference of the presently read voltage V from the previous voltage V0by an infinitesimal time Δt. The infinitesimal time Δt is set as a timeconsumed between the reading of the voltage V performed in step S301when the current I equals the predetermined current Iset and the readingof the voltage V in step S306 as in the routine illustrated in FIG. 4.

After determining the differential value dV/dt, the CPU 62 determines instep S310 whether the differential value dV/dt is within a properhumidification range set between a threshold γ and a threshold δ. Whenthe amount of oxidative gas supplied is increased in step S300, thewater evaporation from the electrolyte membranes 32 is accelerated. Whenthe humidification of the electrolyte membranes 32 is insufficient, theproton conductivity of the electrolyte membranes 32 does notsignificantly change even if the water evaporation from the electrolytemembranes 32 is accelerated to bring about further insufficienthumidification. In that case, therefore, the differential value dV/dtbecomes small. Conversely, when the humidification of the electrolytemembranes 32 is excessive, acceleration of the water evaporation fromthe electrolyte membranes 32 brings the condition closer to properhumidification, so that the proton conductivity of the electrolytemembranes 32 increases. In that case, therefore, the differential valuedV/dt becomes great. Therefore, the humidification condition of theelectrolyte membranes 32 can be determined by determining a differentialvalue dV/dt when the humidification condition of the electrolytemembranes 32 is within the proper range, and setting a properhumidification range through the use of the threshold γ and thethreshold 6 as a lower limit and an upper limit, respectively, andcomparing the calculated differential value dV/dt with the properhumidification range.

When the differential value dV/dt is within the proper humidificationrange, the CPU 62 determines in step S312 that the humidification of theelectrolyte membranes 32 is proper (proper humidification) . When thedifferential value dV/dt is equal to or less than the threshold γ, theCPU 62 determines in step S314 that the humidification of theelectrolyte membranes 32 is insufficient (insufficient humidification).When the differential value dV/dt is equal to or greater than thethreshold δ, the CPU 62 determines in step S316 that the humidificationof the electrolyte membranes 32 is excessive (excess humidification).This routine then ends.

By executing the humidification condition determining routineillustrated in FIG. 5, the fuel cell system 20 is able to determine ahumidification condition of the electrolyte membranes 32 based on thevoltage V and the current I outputted by the fuel cell 30 when theamount of oxidative gas supplied to the fuel cell 30 is increased.

In the foregoing routine, the amount of fuel gas supplied to the fuelcell 30 maybe increased with increases in the amount of oxidative gassupplied to the fuel cell 30.

Next described will be a humidification condition determining routineillustrated in FIG. 6. When this routine is started, the CPU 62 firstreads a voltage V detected by the voltmeter 40 in step S400.Subsequently in step S402, the CPU 62 substitutes the read voltage V forthe previous voltage V0. After increasing the amount of oxidative gassupplied to the fuel cell 30 in step S404, the CPU 62 reads a voltage Vagain in step S406. Subsequently in step S408, the CPU 62 calculates avoltage deviation ΔV by subtracting the previous V0 from the readvoltage V.

Subsequently in step S410, the CPU 62 checks whether the voltagedeviation ΔV is a negative value. When the amount of oxidative gassupplied to the fuel cell 30 is increased, the water evaporation fromthe electrolyte membranes 32 accelerates. However, if the electrolytemembranes 32 are properly humidified, acceleration of the evaporationfrom the electrolyte membranes 32 up to a certain extent does notsignificantly change the voltage V outputted from the fuel cell 30. Ifit is found in step S410 that the voltage deviation ΔV is equal to orgreater than “0”, the CPU 62 determines in step S412 that thehumidification of the electrolyte membranes 32 is proper (properhumidification). If the electrolyte membranes 32 are insufficientlyhumidified, an increase in the amount of oxidative gas supplied to thefuel cell 30 exacerbates the insufficiency in the humidification of theelectrolyte membranes 32, so that the voltage V outputted from the fuelcell 30 decreases even further. Therefore, if it is found in step S410that the voltage deviation ΔV is less than “0”, the CPU 62 determines instep S414 that the humidification of the electrolyte membranes 32 isinsufficient (insufficient humidification). The routine then ends.

By executing the humidification condition determining routineillustrated in FIG. 6, the CPU 62 is able to determine a condition ofhumidification of the electrolyte membranes 32 based on the voltage Voutputted from the fuel cell 30 when the amount of oxidative gassupplied to the fuel cell 30 is increased.

Although only the amount of oxidative gas supplied to the fuel cell 30is increased in the humidification condition determining routineillustrated in FIG. 6, it is also possible to increase the amount offuel gas supplied to the fuel cell 30 with an increase in the amount ofoxidative gas supplied to the fuel cell 30.

Next described will be a humidification condition determining routineillustrated in FIG. 7. In order to execute this routine, the voltmeter40 of the fuel cell 30 needs to detect the voltage of each unit cell 31.The below description will be made, assuming that the voltmeter 40detects the voltage V of each unit cell 31 of the fuel cell 30.

When the routine illustrated in FIG. 7 is started, the CPU 62 firstreads the voltage V of each unit cell 31 of the fuel cell 30 detected bythe voltmeter 40 in step S500. Subsequently in step S502, the CPU 62calculates a variance σ0 of the read voltages V of the individual unitcells 31. After increasing the amount of oxidative gas supplied to thefuel cell 30 in step S504, the CPU 62 again reads the voltage V of eachunit cell 31 detected by the voltmeter 40 in step S506. Subsequently instep S508, the CPU 62 calculates a variance σ1 of the read voltages V ofthe unit cells 31.

Subsequently in step S510, the CPU 62 compares the calculated variancesσ0 and σ1. When the amount of oxidative as supplied to the fuel cell 30is increased so as to accelerate water evaporation from the electrolytemembranes 32, the condition of the electrolyte membranes 32 changes froman excessively humidified condition toward a properly humidifiedcondition, so that the variation (variance) of the voltages V of theunit cells 31 decreases. Therefore, if the variance σ0 is equal to orless than the variance σ1, the CPU 62 determines in step S512 that theelectrolyte membranes 32 are properly humidified (properhumidification). If the variance σ0 is greater than the variance σ1, theCPU 62 determines in step S514 that the electrolyte membranes 32 areexcessively humidified (excess humidification). The routine then ends.

By executing the humidification condition determining routineillustrated in FIG. 7, the CPU 62 is able to determine a condition ofhumidification of the electrolyte membranes 32 based on the variation ofthe voltages V of the individual unit cells 31 of the fuel cell 30occurring before and after an increase in the amount of oxidative gassupplied to the fuel cell 30.

In the above-described routine, the amount of fuel gas supplied to thefuel cell 30 may also be increased with an increase in the amount ofoxidative gas supplied to the fuel cell 30.

Furthermore, if the above-described routine is applied to a fuel cellsystem including a plurality of modules each of which is formed by aplurality of unit cells, it is also possible to determine a condition ofhumidification of the electrolyte membranes 32 based on the variation(variance) of the voltages outputted by the individual modules.

While the present invention has been described with reference to what ispresently considered to be a preferred embodiment thereof, it is to beunderstood that the invention is not limited to the disclosed embodimentor constructions. To the contrary, the invention is intended to covervarious modifications and equivalent arrangements.

What is claimed is:
 1. A fuel cell system comprising: a polymerelectrolyte type fuel cell formed by stacking unit cells, each of whichhas an electrolyte membrane sandwiched by two electrodes; a fuel gassupply device connected to the fuel cell; a fuel gas humidifier; a fuelcell output electric current detector; a fuel cell resistance detector;and a humidification condition determiner that determines a condition ofhumidification of the electrolyte membranes based on the currentdetected by the current detector and the resistance detected by theresistance detector, wherein the humidification condition determinerdetermines a condition of humidification of the electrolyte membranesbased on an over-time change in the resistance detected by theresistance detector when the current detected by the current detector isequal to a predetermined value.
 2. A fuel cell system according to claim1, wherein if the over-time change in the resistance is equal to or lessthan a first predetermined value, the humidification conditiondeterminer determines that the electrolyte membranes are insufficientlyhumidified, and wherein if the over-time change in the resistance isequal to or greater than a second predetermined value that is greaterthan the first predetermined value, the humidification conditiondeterminer determines that the electrolyte membranes are excessivelyhumidified.
 3. A fuel cell system comprising: a polymer electrolyte typefuel cell formed by stacking unit cells, each of which has anelectrolyte membrane sandwiched by two electrodes; a fuel gas supplydevice connected to the fuel cell; a fuel gas humidifier; a fuel celloutput electric current detector; a fuel cell resistance detector; and ahumidification condition determiner that determines a condition ofhumidification of the electrolyte membranes based on the currentdetected by the current detector and the resistance detected by theresistance detector, further comprising a humidification controller thatcontrols humidification in the fuel gas caused by the fuel gashumidifier, based on determination made by the humidification conditiondeterminer.
 4. A fuel cell system according to claim 3, wherein thehumidification controller increases the humidification in the fuel gasif the humidification condition determiner determines that theelectrolyte membranes are insufficiently humidified.
 5. A fuel cellsystem according to claim 3, wherein the humidification controllerincreases a pressure of the fuel gas in the fuel cell if thehumidification condition determiner determines that the electrolytemembranes are insufficiently humidified.
 6. A fuel cell system accordingto claim 3, wherein the humidification controller decreases an operatingtemperature of the fuel cell if the humidification condition determinerdetermines that the electrolyte membranes are insufficiently humidified.7. A fuel cell system according to claim 3, wherein the humidificationcontroller decreases an amount of the fuel gas supplied by the fuel gassupplier if the humidification condition determiner determines that theelectrolyte membranes are insufficiently humidified.
 8. A fuel cellsystem according to claim 3, wherein the humidification controllerdecreases the humidification in the fuel gas if the humidificationcondition determiner determines that the electrolyte membranes areexcessively humidified.
 9. A fuel cell system according to claim 3,wherein the humidification controller decreases a pressure of the fuelgas in the fuel cell if the humidification condition determinerdetermines that the electrolyte membranes are excessively humidified.10. A fuel cell system according to claim 3, wherein the humidificationcontroller increases an operating temperature of the fuel cell if thehumidification condition determiner determines that the electrolytemembranes are excessively humidified.
 11. A fuel cell system accordingto claim 3, wherein the humidification controller increases an amount ofthe fuel gas supplied by the fuel gas supplier if the humidificationcondition determiner determines that the electrolyte membranes areexcessively humidified.
 12. A fuel cell system according to claim 3,further comprising an abnormality detector that detects an abnormalityof the fuel cell system when the determination made by thehumidification condition determiner remains unchanged after control bythe humidification controller has been performed for a predeterminedamount of time.
 13. A fuel cell system according to claim 12, furthercomprising a detected abnormality information outputter.
 14. A fuel cellsystem according to claim 12, further comprising a device that stops anoperation of the fuel cell system when the abnormality detector detectsthe abnormality.